Prevention of corrosion in aqueous systems



Patented Jan. 8, 19 52 PREVENTION OF CORROSION IN AQUEOUS SYSTEMS Arthur L. Jacoby, Western Springs, 111., assignor to National Aluminate Corporation, Chicago, 111., a corporation of Delaware No Drawing. Application September 13, 1948, Serial No. 49,123

9 Claims.

This invention relates to the-prevention of corrosion by aqueous heat transfer media for use in heat exchange devices and the like, such as the cooling systems of internal combustion engines, oil coolers, and other heat exchangers, particularly those devices containing, in addition to ferrous metals, aluminum in contact with copper, brass, or similar alloys.

The corrosive action of aqueous solutions on metals presents an important problem in various industries. One instance in which the problem has become particularly important is the use of natural waters, or softened or deionized Waters as heat transfer media in cooling systems associated with internal combustion engines, including not only the solutions used in the engine jackets but also those employed in auxiliary heat exchange devices, such as, for example, oil coolers. Corrosion of the metals of such heat exchange systems is, of course, undesirable, not only because it may cause actual failure of cylinder walls, but particularly because the products of corrosion freqently tend to clog the smaller passages of the system, impairing the circulation. At other times the corrosion products may adhere to the walls of the system in such a way that they seriously reduce the rate of heat transfer.

The number of attempted solutions to this problem has been large. Many materials have been suggested and used in an effort to reduce the corrosive tendencies of Waters toward the common cooling system metals. Some of these materials have met with moderate success in that they have provided substantial protection for certain metals, but most of them fall short in their capacity to adequately protect all parts of the cooling systems commonly encountered. Especially is this true in the case of systems in which aluminum, in contact with copper, brass, Admiralty metal, or other similar alloy, occurs.

One of the objects of the present invention is to provide an aqueous heat transfer medium which is substantially non-corrosive to ferrous metals and other metals commonly encountered in heat transfer systems, including aluminum in contact with copper, brass, Admiralty metal, and other similar alloys.

Another object of this invention is to provide a composition which, when added to natural, softened, or deionized water, will render it substantially non-corrosive to ferrous and other metals, particularly aluminum, in contact with copper, brass, Admiralty metal, and other similar alloys.

A still further object is to provide a composition, as set forth above, which will also furnish a control of the pH of the system within a de sired range. Other objects will appear hereinafter.

In accordance with this invention, it has now been found that the ferrous metals, and also aluminum and aluminum alloys in contact with copper and copper alloys, e. g., brass, Admiralty metal, and similar alloys, are very effectively protected from the corrosion of aqueous solutions in contact therewith when said solutions contain soluble chromate and soluble nitrate at a pH of, preferably, 6.5 to 9.5. Under comparable experimental or operating conditions an aluminum-copper alloy couple is not adequately protected by soluble chromate alone, or by soluble nitrate alone, in spite of the fact that chromate is widely and successfully used for protecting ferrous metals, and soluble nitrates, under certain conditions, are known to protect aluminum.

While the concentrations of the two inhibitors, required to adequately protect the aluminum in an aluminum-alloy couple, will vary somewhat with the corrosiveness of the water and with the hardness and other characteristics thereof, it has been found that a water soluble chromate in the range of 500 to 3000 parts per million (P. P. M.) (0.05% to 0.30% by weight), expressed as NazCrzOmZHzO, and a water soluble nitrate in the range of 250 to 1000 parts per million (0.025% to 0.10% by weight), expressed as NaNOz, is adequate when the pH of the solution is within the approximate range of 6.5 to 9.5. The protection afforded ferrous metals by chromate in this preferred concentration range is well known. However,'as stated above, neither the chromate by itself nor the nitrate by itself, in the ranges given here, will render the waters non-corrosive to the aluminum of the aluminumalloy couple. These facts may be further illustrated by some exemplary test results, tabulated A cylindrical test specimen having overall dimensions of approximately 3 inches in length by 0.225 inch diameter was constructed in the following way: A piece of aluminum of the type commonly employed in heat exchangers Al and 5% 1 i'nch was 0.625 inch long and the remaining halfinch was reduced to a diameter of about 0.075 inch. A cylinder of Admiralty metal (70% Cu, 29% Zn, and 1% Sn) of length approximately assaiee 4 experienced due to the corrosion of aluminum in such bi-metallic couples even when 3000 to 4000 parts per million of chromate was present in the aqueous coolant, and such failures eliminated by 2 inches and diameter 0.225 inch was then axially 5 the use of the combination of chromate and bored to receive, with a snug fit, the smaller end nitrate contemplated by this invention. of the aluminum cylinder. The resulting speci- The efiect of pH on corrosion is well known and men was a smooth cylinder approximately 3 3 it is preferable, in most corrosion control procinches in total length, containing the two metals esses, to regulate the pH of the aqueous heat in intimate contact. This specimen was then transfer medium. The preferred pH range for mounted vertically in the chuck of an electric the present invention is 6.5 to 9.5 and a premotor so that the aluminum end was down, and ferred embodiment of the invention contemplates immersed to a depth of 1.25 inches in the soluthe provision of a composition similar to that tion to be tested. The specimen was rotated durshown last in the table. The sodium carbonate ing the test at a speed of 1750 revolutions per and borax provide the alkalinity necessary to minute. overcome the acidity of the dichromate and a .The solution employed in these tests was made solution of the composition shown in the table, up of a 1:1 (by volume) mixture of Chicago at a concentration of 3000 parts per million (by tap water and distilled water, with sodium chloweight) in distilled water has a pH of 6.78, while ride added to increase the sodium chloride cona solution of the same strength in Chicago tap tent by 10 grains per gallon. All tests were run water has a pH of 6.84. Due to the bufiering at 150 i 5 F. in a volume of 550 i- 10 milliliters action of the borax, these solutions, upon being of solution. The inhibitors indicated in the table heated for 340 hours at 150 F. to 160 F., changed were dissolved in the test solution before the start only to pH values of 6.80 and 7.26, respectively. of the test. At the end of the test, the specis many wat r s lubl chromate or dimen was visually examined and the aluminum in chromate, i c d n t O e Of t e a kali metals, the test solution determined. ammonia, etc., and any water soluble nitrate salt,

Table Dose, P. P. M. Percent Average Qhange Inhibit Om ea. esh- Total mate, NaN 0;; Mils/Yr. glgirk None 0 0 0 96 8.2-9.5 18.3 2, 000 0 2,000 330 9. 09.5 23.1 +20 NaNOa 4, 000 0 4, 000 330 9.0-9.5 30.0 +04 7, 000 0 7, 000 336 9.0-9.5 20.9 +47 10.000 0 10,000 336 9. 0-9.5 23.7 920 920 0 194 9.3-9.5 11.9 Chromate, 4, 590 4,590 0 194 8.5-9.5 12.9 -30 10,000 10,000 0 194 8.6-9.5 12.9 -30 ,g g f 1, 420 920 500 381 8.5-9.5 3.0 80 251%" 1,920 920 1,000 38l 8.7-9.6 5.9 -19 60% chromate. g2 992 its 20 is ii z 7 i2 5%Bomx 0, 000 9,590 1,200 399 9. 4-9.5 0.2 -00 1 Chromate throughout this table is expressed as Na2Cr O12H O.

From the results shown in the table, it is apare equally-suitable for the practice of this invenparent that the aluminum of the aluminum-alloy 60 tion; The term "water soluble refers to water couple is severely attacked in the absence of any solubility at least sufiicient to cause solution of inhibitor. Although, under some conditions, the chromates and nitrates in the proportions of nitrates are recommended for the protection of 500 parts per million and 250 parts per million, aluminum, the results of the tests show that the respectively. 5 Y 1 aluminum of the couple is even more severely The invention is hereby claimed as follows: corroded when the only treating agent is sodium 1. A noncorrosive aqueous solution consisting nitrate, up to concentrations of NaNOs as high essentially of water and approximately 1 800-parts as 10,000 parts per million (1.0% by weight). p r million, y Weight, of chromate, as Additionally, sodium nitrate employed alone 1 would not protect the other metals in the cooling 1 NazCmOLZI-ko 1 system. and 600 parts per million, by weight, of nitrate,

Chromate, under the test conditions, also does as NaNOaand having a pH in the range 6.5 to 9.5. not adequately protect the aluminum in the 2. A corrosion inhibiting composition adapted aluminum-alloy couple, although it is true that to be added to an aqueous fluid which contacts chromate, in the concentration range tested, will metallic surfaces, said composition consisting esadequately protect the ferrous metals in the syssentially of a water soluble chromate and a water tem. However, the combination of chromate and soluble nitrate in relative weight proportions of nitrate exhibitsv the remarkable pr p r y f v ry 500 to 3000 parts of the water soluble chromate, greatly reducing the corrosion of the aluminum expressed as Na2Cr2Om2I-I2O', and 250 to 1000 in the aluminum-alloy couple. As illustrated, parts of the water soluble nitrate, expressed as moreover, the result is obtained with a fraction NaNOg, said composition being effective to inhibit f a p cent of total chromate and nitrate based corrosion in aqueous solutions in contact with p the W t 0 t e solution treated. aluminum and copper when present-in said soluh above observations are a borne t in tions in proportions corresponding to 0.05 to 0.3% fi p ac ice where Seri u s have been rxby weight of water soluble chromate, expressed,

as NazCr2O7.2HzO, and 0.025 to 0.1% by weight of water soluble nitrate, experssed as NaNOa.

3. A corrosion inhibiting composition for addition to an aqueous heat transfer medium which contacts metallic surfaces, said composition consisting essentially of sodium dichromate and sodium nitrate in relative weight proportions of 500 to 3000 parts of sodium dichromate to 250 to 1000 parts of sodium nitrate, said composition being effective to inhibit corrosion in aqueous solutions in contact with aluminum and copper when present in said solutions in proportions corresponding to 0.05 to 0.3% by weight of sodium dichromate and 0.025 to 0.1% by weight of sodium nitrate.

4. An aqueous solution having a pH within the range of 6.5 to 9.5, inhibited against corrosion by 500 to 3000 parts per million of a water soluble chromate, expressed as Na2Cr2Om2H2O, and 250 to 1000 parts per million of a water soluble nitrate.

5. A corrosion inhibiting composition comprising essentially 64.7% by weight water soluble chromate, expressed as NazCr2Om2HzO, and 35.3% by weight sodium nitrate, said composition being effective to inhibit corrosion in aqueous solutions in contact with aluminum and copper when present in said solutions in proportions corresponding to 0.05 to 0.3% by weight of Water soluble chromate, expressed as Na2Cr2Om2I-I2O, and 0.025 to 0.1% by weight of water soluble nitrate, expressed as NaNOg.

6. A corrosion inhibiting composition comprising essentially 47.3% by weight water soluble chromate, expressed as Na2Cr2O7.2H2O, and 52.2% by weight sodium nitrate, said composition being effective to inhibit corrosion in aqueous solutions in contact with aluminum and copper when present in said solutions in proportions corresponding to 0.05 to 0.3% by weight of water soluble chromate, expressed as Na2Cr2O7.2H2O, and 0.025 to 0.1% by weight of water soluble nitrate, expressed as NaNOa.

7. A corrosion inhibiting composition comprising essentially 60% by Weight water soluble chromate, expressed as NazCrzOmZI-hO, 20% by weight sodium nitrate, 15% by weight sodium carbonate and 5% by weight borax, said composition being efiective to inhibit corrosion in aqueous solutions in contact with aluminum and copper when persent in said solutions in proportions corresponding to 0.05 to 0.3% by weight of water soluble chromate, expressed as NazCrzOmZI-IzO and 0.025 to 0.1% by weight of water soluble nitrate, expressed as NaNOa.

8. A method of inhibiting corrosion in aqueous solutions in contact with both aluminum and copper which comprises incorporating with said solutions 500 to 3000 parts per million of a water soluble chromate, expressed as REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number a Name Date 1,804,463 Erickson May 12, 1931 1,827,223 Dennis Oct. 13, 1931 1,927,842 McDermott Sept. 26, 1933 1,953,003 Michel Mar. 27, 1934 2,141,049 Skeen Dec. 20, 1938 2,153,952 Bayes Apr. 11, 1939 2,411,676 Burghart Nov. 26, 1946 

2. A CORROSION INHIBITING COMPOSITION ADAPTED TO BE ADDED TO AN AQUEOUS FLUID WHICH CONTACTS METALLIC SURFACES, SAID COMPOSITION CONSISTING ESSENTIALLY OF A WATER SOLUBLE CHROMATE AND A WATER SOLUBLE NITRATE IN RELATIVE WEIGHT PROPORTIONS OF 500 TO 3000 PARTS OF THE WATER SOLUBLE CHROMATE, EXPRESSED AS NA2CR2O7.2H2O, AND 250 TO 1000 PARTS OF THE WATER SOLUBLE NITRATE, EXPRESSED AS NANO3, SAID COMPOSITION BEING EFFECTIVE TO INHIBIT CORROSION IN THE AQUEOUS SOLUTIONS IN CONTACT WITH ALUMINUM AND COPPER WHEN PRESENT IN SAID SOLUTIONS IN PROPORTIONS CORRESPONDING TO 0.05 TO 0.3% BY WEIGHT OF WATER SOLUBEL CHROMATE, EXPRESSED AS NA2CR2O7.2H2O, AND 0,025 TO 0.1% BY WEIGHT OF WATER SOLUBLE NITRATE, EXPRESSED AS NANO3. 